Some Queries

I have some very important WordPress questions to ask for my upcoming website project. Will appreciate the answers. I am using the Platform theme, here is the screenshot:
https://imageshack.us/photo/my-images/27/unledhmj.png/

1. As you can see in the screenshot, I have managed to decrease the height of the page and the black background is visible, I want to do the same with the width so that I can see the background from left and right side. How can I do it?

2. Can I add a widget in the background header (black space at the top), that is because I want to add a banner image, some links and box size images up there?

3. How can I specify borders to posts area and change posts area background color?

4. Lastly, I have a WordPress Network that have different blogs for each type of content, lets say Blog 1 for Content 1, Blog 2 for Content 2, and so on. Now I would want to have the main page of the site to display widget of each blog, showing the latest posts. Lets say I have https://www.abc.com as my main site and https://www.abc.com/blog1 and https://www.abc.com/blog2 as sub-sites within a network. Now I want https://www.abc.com to have two widgets, the first widget will show the latest posts from https://www.abc.com/blog1 and second widget will show the latest posts of https://www.abc.com/blog2. How can I achieve this?

Thanks a lot in advance!